The present invention is directed to an inductive motion sensor that can be attached to a subject, the inductive motion sensor having a coil and a magnet member movable relative to the coil.
The function of such motion sensors is based on the fact that dislocations of the magnet member relative to the coil occur as a result of motions of the subject to which the motion sensor is attached, thereby inducing an electrical voltage in the coil. When the motion sensor is attached to a life form in order to monitor its physical activity, the induced current is interpreted in terms of its amplitude and/or chronological change in order to be able to draw conclusions regarding the degree and/or nature of the physical activity of the life form. When, by contrast, the motion sensor is used as an anti-theft sensor, then it is generally adequate to detect the mere occurrence of an induced voltage in order to be able to determine that the article to be protected is being removed from its location.
In known motion sensors of the type initially set forth, the problem arises that due to their high sensitivity, i.e. their response to extremely small motions, disturbances occur as a result of the magnetic field of the earth or as a result of low-frequency magnetic fields emanating from electrical devices, for example electric motors. Erroneous detections often occur particularly given the occurrence of low-frequency magnetic fields, i.e. motions of the magnet member relative to the coil occur without a motion of the subject to be monitored being present.